The invention relates to an evaluation method for ECG mapping, in which a plurality of electrodes attachable to the thorax of the patient at known distance from the heart are used, and in which time-dependent parameters that specially accentuate the multipole components of the cardiac electrical field are calculated from the voltages at the electrodes.
In cardiological diagnostics, a technique known as ECG mapping, in which a plurality of electrodes for detecting cardiac potentials are disposed on the thoracic surface, is used to represent cardiac potentials. In this technique, it is known how to represent the cardiac potentials recorded with the electrodes on the thoracic surface imagined to be unrolled as a flat figure (H. Abel: Electrocardiology 19888, Excerpta Medica, Amsterdam, pp. 357-360). This technique has the disadvantage that it fails to consider the different distances of the lead electrodes from the electrical center of the heart and the influence thereof on the electrical field.
In order to consider the influence of thoracic configuration on the potential field measured at the thoracic surface, the potential field at the cardiac surface is calculated from that potential field. Such methods are known as solutions of the "inverse problem". Either a model surface of the heart is assumed (IEEE Transactions on Biomedical Engineering, Vol. 31 (1984), No. 9, pp. 611-621) or the shape of the heart can be determined by means of NMR tomography (NMR=nuclear magnetic resonance).
Instead of representing the potential field as an intensity field, isochrones on the cardiac surface are also plotted both for model hearts (IEEE Transactions on Biomedical Engineering, Vol. 31 (1984), No. 10, pp. 652-659) and for heart shapes measured by tomography (IEEE Transactions on Biomedical Engineering, Vol. 35 (1988), No. 12, pp. 1047-1058).
The calculation of the potential distribution or of isochrones on the surface of a theoretical heart shape can lead to considerable misinterpretations, if the shape and position of the model heart differ from those of the actual heart. This method is suitable only for theoretical studies. Measurement of the actual heart shape by NMR tomography can be life-threatening for patients with metallic implants, such as pacemaker patients and heart-transplant patients with IMEK electrodes. The actual shape and position of the heart cannot be determined in this way in such patients. In other patient groups, considerable difficulties occur routinely in coupling the image transmission from the tomograph to the ECG mapping apparatus.
A known proposal is to project the ECG curves measured on the thorax onto a sphere around the electrical center of the heart ("Problems of three-dimensional vectorcardiography", Int. Coll. Stary Smokovec, 1961. Publishing House of the Slovakian Academy of Sciences, Bratislava, 1963, pp. 43-54). The projection is made along the lead lines from sphere center to electrode corresponding to the dipole hypothesis. The diagrammatic representation of complete ECG curves at the projection points on the spherical surface is unclear, has poor resolution in time and amplitude and does not contain data reduction for compressed information, and so is unsuitable for routine diagnosis.
Furthermore, the representation of ECG curves, equipotential lines or isochrones as in the method described in the foregoing is not suitable, regardless of the representation surface, for specially marking perturbation boundaries of local perturbations of field propagation, because local amplitudes must always be assessed as a proportion of the total state of excitation. At the measuring point under the electrode, the potential is always a summation potential produced by all excited locations in the heart. Locally confined perturbations, which are represented in particular in the higher multipole components, are therefore suppressed.
In order specially to represent the higher multipole components, a proposal is known (Electrocardiology '87, AkademieVerlag Berlin 1988, pp. 165-167) according to which the voltages of Frank's voltage vector projected to an electrode location are subtracted from the voltage measured thereat. The projection of Frank's voltage vector is undertaken using a transformation matrix, which was obtained statistically from comparisons of ECG curves. A falsifying effect is introduced by the fact that the same matrix is used regardless of the thoracic configuration of the subjects. Since the thoracic shape also is considered neither individually nor as a mean for the measured voltage, the difference between the measured voltage and the projection of Frank's voltage vector is also not constant for a dipole field, and the effect of higher multipoles is superposed on the influence of thoracic configuration, thus leading to severe distortions of the isopotential lines and making comparisons between patients difficult.